Is this an option that's available all the time? Back when the Nexus 5 was still new, I tried doing exactly this and took one I just bought to my local Verizon outlet to have it activated on their network. The tech there told me that while the phone DID have the hardware needed to be compatible on Verizon's network, he was "not allowed" to activate it for me unless I bought it directly through them. I've since switched to T-Mobile and never looked back.
The car salesman will also try to tell you he's not allowed to let you take the vehicle off the lot without rustproofing.
Verizon around the Nexus 5 still had a lot of CDMA and you needed to have a phone activated.
Now that networks are pretty much LTE and 5G only, if your phone takes a SIM, take the SIM out of the old phone and put it in the new phone. Some carriers still play games with allowlists for VoLTE though.
But you might have better luck (and better pricing) with a MVNO or the prepaid side of your preferred carrier.